Preparation for smelting of metals and compounds with high melting points



PREPARATION FOR SMELTING OF METALS AND COMPOUNDS WITH HIGH MELTING POINTS Filed Feb. 17, 1965 July 2, 1968 P. HIMMELSTEIIN ET l- 3,391,238

United States Patent 4 Claims. (cl. 13-9) ABSTRACT OF THE DISCLOSURE An arc furnace for melting materials of high melting point. The furnace comprises a crucible having a bottom portion insulated from the remainder of the crucible which serves as a lower electrode, an upper electrode mounted axially above the lower electrode and a means for establishing an are between the electrodes. A bottom electrode insulated in this manner aids in providing for a stabilized arc.

This invention relates to the smelting of metals, alloys, and compounds having high melting points. More particularly, this invention discloses an arc furnace in which metals and compounds having a high melting point are smelted by the skull-melting process.

In the metallurgical process of smelting metals, alloys and compounds having high melting points, furnaces are used in which the energy required for smelting is applied in the form of either a concentrated electron beam or an electric arc. Electron beam furnaces can be operated only in a high vacuum or, when using an electron gun, in a medium vacuum. On the other hand, are furnaces can operate either in a vacuum, when the plasma of the arc consists exclusively of the vapor from the melt, or under a inert gas at any desired pressure, when the plasma consists mainly of ionized gas. The second method is particularly suitable when alloys or compounds to be smelted have a component which is slightly volatile at the smelting temperature. The inert gas cannot eliminate their undesired volatility, but it can diminish it.

As a mic, no crucible material is available for the smelting of metals and compounds having high meltingpoints which does not react with the melt. However, the reaction of the crucible with the melt can be prevented by a special smelting technique called skull-melting. In this process the melt is contained in a cooled crucible, thereby procuring a sharp temperature drop between the melt and the crucible so that a solid crust of the melt prevents direct contact between the melt and the crucible.

In the present forms of arc furnace construction, a copper crucible is connected at a suitable point by mechanical fastening to one pole of the power source. The path of the current from the point of contact on the crucible to the point of contact of the are on the melt is then left to chance. The size of the molten pool influences the thickness of the skull, and the force of pressing the skull against the cooled crucible depends upon its temperature and it expansion coefficient. In this way points of contact arise betwen skull and crucible which may be distributed perferentially at the side. Because of the cross flow of the current across the melt, the magnetic field accompanying the current is also distorte'd, and this also has adverse effects upon the arc. As the contact-points slowly wander, the arc moves about on the molten pool in an uncontrolled way accordingly. The smelting of the material thus occurs very unevenly and the total quantity melted may vary greatly under otherwise identical conditions.

The primary objectof this invention, therefore, is to provide an improved arc furnace for melting metals and compounds having high melting points.

Another object of this invention is to provide a stabilized arc for a high temperature are smelting furnace.

Other and further objects will become apparent from the following detailed description and the accompanying drawings wherein:

FIG. 1 illustrates an arc furnace in which the bottom electrode is inserted in the center of the crucible floor and insulated therefrom, and

FIG. 2 shows an arc furnace in which the bottom electrode is the crucible floor and is insulated from the sides of the crucible.

Referring now to the drawing, FIG. 1 shows a copper crucible 1 having a diameter of 180 mm. and a height of mm. [t is cooled by a coil 2 through which water flows. The crucible is surrounded by coil 3 with which a magnetic field of at least 2000 ampere turns can be generated. In the central axis of the crucible is an electrode 4 which is insulated from the remainder of the crucible by an insulating sheath 5. Above the crucible is a graphite electrode 6 which is secured in a Watercooled holder. The electrode has an axial boring 7 which grows with use into a concave depression 8. The insulated electrode 4 in the crucible is connected to the plus pole of a high-current rectifier and the electrode 6 to the minus pole. The entire smelting apparatus is located in a sealed argon containing vessel which is not illustrated.

FIG. 2 shows an alternative design in which the entire floor 9 acts as an electrode and is insulated from the wall 11 by a ring shaped insulating material It The material 12 in the corners of the crucible is part of the unsmelted charge. In this design, too, the entire current flows in an axial direction from the bottom of the crucible through the skull 13 and the melt 14 to the electrode 15.

It has been found that the disadvantage of the prior art are furnaces could be avoided by electrically insulating the walls of the crucible from the current supply to the bottom electrode. The current supply can be pro vided over the whole electrically-conducting floor surface, or by an electrode in the middle of the floor insulated from the remaining parts of the crucible, its diameter being small compared with the bottom of the crucible. With this design, the entire current can be directed axially from the contact-point on the bottom of the crucible through the skull, the melt and then through the plasma to the electrode. The size, number and position of the contact points between the skull and the insulated crucible wall may then vary at random without influencing current paths. Small deviations of the current from the axis and corresponding deviations of the arc can then be influenced and corrected by an external magnetic field. The magnetic liel-d is created by a coil surrounding the crucible and is arranged so that the central axis of the field coincides with the ideal current path. In this arrangement no-vertical forces arise, but if the arc deviates somewhat tangential forces arise in accordance with the basic electrodynamic rule F =H XI, where F is force, H is the intensity of the magnetic field, and I is current. This application of magnetic force causes rotation of the melt which increases the quantity of the melted substance and mixes it more thoroughly at the same time; moreover it avoids an excessively steep temperature drop between the surface of the melt and the contact area between the melt and the skull.

As a result of fixing of the arc on the middle of the melt, the electrode burns out from the middle. The cathode burning spot stabilizes itself in the middle of the electrode so that there is an appreciable burning away at this point due to the higher temperature. When the magnetic field is stabilized, the burn area on the electrode increases and the current load diminishes accordingly, thus the inevitable erosion of the electrode is reduced to a minimum and may be considered to be almost constant. Since the loss of material at the electrode is partly taken up by the melt, it can be seen that as a result of the stabilization of the arc and the increase of the cathode burn area, the impurities in the melt are also reduced. Because of the beneficial symmetry of the are secured by the above arrangements on the crucible, permanent use leads to a concave depression on the electrode where the arc burns. The formation of the depression can be assisted in a simple manner by boring a hole in the middle of the electrode. The electrode burning commences at the edge of the hole and extends to form the concave depression in the course of use.

Example Broken pieces of uranium. monocarbide weighing 15 kg. were placed in the crucible. The arc was started by contacting the graphite electrode with the uranium monocarbide. The distance between the melt and the electrode was regulated so that the potential drop is in the order of 35 volts. When the magnetic centering field was switched on, the melt began to rotate slowly owing to slight asymmetries without diverting the are from the center of the melt. After the end of the melt program which was finished in 7 minutes at 3000 amperes, the electrode was drawn up and the molten uranium monocarbide was poured out of the crucible through a spout into a mold and cooled.

With the arc furnace according to this invention, it is possible to carry out a large number of smeltings one after the other in the same vessel. The same amount of material is smelted each time with good reproducibility. With the arc furnace of this invention, the charge material is evenly heated, and the wall of the crucible has no influence on the smelting process.

It is to be understood that certain changes and modifications can be performed on the apparatus disclosed herein without departing from the spirit and scope of the invention as encompassed by the following claims.

We claim:

1. An improved arc furnace for skull melting materials of high melting point comprising:

(a) a crucible having a bottom portion serving as a lower electrode, the lower electrode portion being electrically insulated from the remainder of the crucible;

(b) an upper electrode adjustably mounted axially above the lower electrode, and electrically insulated from the crucible; and

(c) means for cooling the crucible.

2. The improved arc furnace of claim 1 wherein the furnace includes means for providing a magnetic field directed axially of the electrodes.

3. The improved arc furnace of claim 2 wherein the upper electrode has an axial base-hole.

4. .The improved arc furnace of claim 1 wherein the upper elect-rode is substantially smaller than the lower electrode.

References Cited UNITED STATES PATENTS 2,789,152 4/1957 Ham et al 13-9 XR 871,338 11/1907 Heroult 13-18 2,727,936 12/1955 Boyer 1331 2,789,150 4/1957 Clough et al 13-9 XR 3,201,560 8/1965 Mayo et al 219123 XR 3,180,916 4/1965 Menegoz et al 13-9 1,833,347 11/1931 Avis 13-9 XR BERNARD A. GILHEANY, Primary Examiner.

H. B. GI'LSON, Assistant Examiner. 

